PocoMail 4. I like it because I can sent out newsletters to a large mailing list as individual Emails. BCC would work in other Email clients, but I can also individualize each Email if I wish using templates.
Jim

While I voted for Kmail as it is what I currently use I am considering switching to something else, not sure if it will be Evolution, Thunderbird or build my own LAMP based system - guess that would be tinkermail then :D

Firefox running Gmail is my choice. I haven't tried many desktop clients (only Thunderbird once) and they would probably work more fluently in some areas, however Gmail has a lot that email clients couldn't offer. Running it on 2 PC's and 3 OS's Gmail is always the same on all of them.

I hope I will find a client that will support server side organization of my messages (labels, stars etc. on Gmail) and will feature what I miss now -- good (kde) desktop integration and ideally interaction with something like Google Talk as well.

...when it was the only one I found that reliably synchronised calendar, contacts and tasks with my old PalmPilot PDA. Right now others do this as well. But while the Thunderbird project has my big sympathy for being the most well-known free software mail client, I'll neither use nor advocate it as long as the Mozilla Corporation has this ghastly attitude of judging theoretical security attacks to be more important than users losing their mails for more than one month. See
https://bugzilla.mozilla.org/show_bug.cgi?id=360409

Kmail. I prefer KDE to Gnome particularly because everything is so well integrated. I use Kontact for work: Calendar, Contacts, Email and RSS all in one package. Works well with Konqueror for web-browsing.

I changed from Thunderbird to Kmail a couple of months ago. Although it's not as friendly as Thunderbird when it comes to html formatting outgoing messages, it has several other advantages for me. The main one is that it is integrated into the Kontact suite. It's nice having my email (Kontact), usenet (Knode), RSS feeds (Akregator), calendar & alarms, address book, to-do's & several other modules all contained in the one program.

I also like the way it auto-detects & integrates Spamassassin, Bogofilter & Clam-AV.

indeed i use claws-mail in Linux and XP.
Test many clients, in both OS must work.
If only for Linux best is kmail, for XP TheBat.
There are not many clients work in both OS so claws-mail is handy, lightweight and fast and handle IMAP.